A method, a circuit arrangement, and a photovoltaic inverter are known from EP 2 372 857 A1. This publication deals with the determination of the fault current component of a differential current, which is determined as a current sum over lines carrying the current of an alternating current generator. For this purpose, an electrical signal is generated by a reference current, caused by voltages at the alternating current generator, through capacitors, to earth. The electrical signal is multiplied by a scaling factor, and the electrical signal, scaled in such a manner, is subtracted, as a measure of a capacitive leakage current component, from the differential current. The scaling factor is continuously adjusted such that the effective value of the differential current is minimized after subtraction of the scaled electrical signal. The subtraction of the scaled electrical signal from the differential current may be effected by means of the summation current transformer that determines the differential current. The effective value of the differential current minimized by subtraction of the scaled electrical signal is the effective value of the fault current, i.e. the differential current still determined by the summation current transformer is the pure fault current component, which can thus be monitored selectively.
It is frequently necessary, in addition to using an RCMU (Residual Current Monitoring Unit), required according to the product standard EN 62109, as a differential current sensor within an electrical device such as, for example, a photovoltaic inverter, to use one or more external RCDs (Residual Current Protection Devices) as additional differential current sensors. These fault current protection means monitor a differential current for conformity with threshold values, but do not normally measure it quantitatively. Even if the compensation of the leakage current component, known from EP 2 372 857 A1, is performed on the differential current within a photovoltaic inverter, this does not prevent erroneous tripping of the external RCDs as a result of high leakage current components of the differential currents monitored by the latter.
It is known from “ABB Technische Information, Fehlerstromschutzschaltung, unerwünschte Auslösungen von Fehlerstromschutzschaltern”, Version 2010 5/C, ABB Schweiz AG, using a three-phase earth-fault circuit interrupter for monitoring of a load with a high leakage capacitance linked between a neutral conductor and one phase conductor. In this case, a compensation capacitor is connected, on the side of the summation current transformer facing back toward the load, between the connection of the phase conductor and a connection of an otherwise unused current path through the summation current transformer. Outside of this summation current transformer the input side of this current path is connected to the connection for the neutral conductor on the output side. In the case of this circuit arrangement an additional capacitive leakage current flows, via the phase conductor, to the compensation capacitor in addition to a differential current resulting from a capacitive leakage current from the load. This additional leakage current flows, besides the phase conductor, twice in the opposite direction through the summation current transformer, namely, on the one hand, via the otherwise unused current path and, on the other hand, via the neutral conductor. In this way, a part of the capacitive leakage current flowing away from the load is compensated, i.e. not determined by the summation current transformer. Limits apply to the compensation capacitor in order that, in the case of a load having been switched off, the leakage current via the compensation capacitor does not trip an earth-fault circuit interrupter comprising the summation current transformer.